Certain normal serum proteins such as transferrin appear to bind to specific sites on human peripheral blood lymphocytes and may be involved in the normal functioning of such cells. Now that the basic properties of transferrin receptors have been established, the plans for the next year of support are focused on the biological importance of these interactions, and particularly their possible roles in the normal functioning of lymphocytes. The effects of transferrin and complexed cation upon lymphocyte activation will be studied, and the stimulatory action already observed will be further examined in an attempt to pinpoint possible mechanisms. One particularly promising lead involves the preliminary observation that occupation of receptors with transferrin causes a substantial rearrangement of related membrane proteins, including the Vitamin D-binding protein Gc, and leads to association with the cytoskeleton. This question will be studied by SDS-PAGE and analytical isoelectric focusing together with transblotting onto nitrocellulose and analysis with specific antisera, both polyclonal and monoclonal. In addition, morphological information will be obtained by immune electron microscopy. Another potentially important observation, namely that lymphocytes release transferrin into the medium within the first few hours of culture, will be further examined by measurement of this protein using a sensitive ELISA method. In addition, the possible contribution of complexed cations, including iron and zinc, and the effects of anti-receptor antibodies in modulating binding of transferrin, will be assessed in terms of lymphocyte function as measured by standard techniques. Finally, recent evidence indicating that covalent modification of membrane proteins, including phosphorylation, is related to binding of ligand will be further investigated. These studies will also examine the effect of the heat-stable placental isoenzyme of alkaline phosphatase that appears on the membrane of activating lymphocytes. Overall, it is hoped that comparative studies of these phenomena in normal lymphocytes, cell lines, and trophoblast will shed further light on the processes involved in cell activation and the requirement for binding of serum proteins such as transferrin. (A)